mineralogical studies of the nitrate american mineralogist, vol. 55, september_october, 1970...

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  • THE AMERICAN MINERALOGIST, VOL. 55, SEPTEMBER_OCTOBER, 1970

    MINERALOGICAL STUDIES OF THE NITRATEDEPOSITS OF CHILE. I I . DARAPSKITE,

    Naa(NOa) (SOn) .HrO'

    GBoncB E. EnrcrsBu aNl Many E. Mnosr, U.S. GeologicalSurtey, Washington, D.C. 20242.

    Ansrnect

    Darapskite, Nas(NOB)(SO{).HzO, though occurring only sparsely at four other locali_ties, is widespread in the nitrate deposits of Chile. It occurs as euhedral crystals in cavitiesand as platy to granular material admixed with other saline minerals. The abundance ofdarapskite here and its scarcity elsewhere results from the exceptionally high concentra-tion of nitrate in the deposits and can be explained by the phase relations in the systemNaNOa-NaCl-NazSOr-HzO, which includes ttre principal saline components of the nitrateore.

    Crystals from Oficina Alemania are monoclinic, P21/m; a:10.564+0.002 A, f :O.Stg+0.001 A, c:5.1890+0.0009 A, p: l}2o 47.gt +0.g,, Z:2, p(meas) 2.202 g/cmt. X-raypowder difiraction data for darapskite, presented for the first time, show the followingstrong lines: 10.29 A (100) (100); 3.a56A (35) (020); 2.365 A (35) (220) and, (O2l);2.59a ir(30) (T02); 4.13 A (25) (210); and 3.522 A e, Qrt). The tine d:r0.2g A is exrremelysensitive and appears in mixtures of saline minerals that contain as little as j of a percentdarapskite. Comparison of physical, optical, crystallographic, and X-ray data confirmspublished data and demonstrates very close similarity between natural and slntheticdarapskite.

    fNrnonucrroN

    This paper is the second of a series of reports describing the physical,chemical, optical, and crystallographic properties, and the distributionand abundance of the saline minerals found in the nitrate deposits ofChile. The fi.rst paper (Mrose and Ericksen, 1970) was an outgrowth ofthe study of darapskitel it shows that the so-called mineral nitroglauber-ite is darapskite. In addition to the discussion of darapskite, the nitratedeposits are briefly described as a background for discussion of mineral-ogy in this and later papers. The mineralogical studies are part of abroad geological investigation that is aimed at determining the origin ofthese unique deposits. The work is being carried out in cooperation withthe Instituto de fnvestigaciones Geol6gicas of Chile.

    Darapskite, Nas(NOr)(SOr).HrO, first described by Djetze (1391)and named for Luis Darapsky of Santiago, Chile, is a widespread mineralin the Chilean nitrate deposits. Only four other occurrences of darapskiteare known: 1) in the Chuquicamata copper deposit (Chile) where itoccurs in veins, as much as 6 inches wide (Jarrell, 1939, p. 634-635),associated with the following minerals: kroehnkite, Na2Cu(SO4)2 . 2HzO;bloedite, NazMg(SOn)r.4HzO; mirabil ite, NagSO+.10HzO; and epso-

    1 Publication authorized by the Director, U. S. Geological Survey

    1500

  • DARAPSKITE 1501

    mite, MgSOa.TH2O; 2) in the nitrate deposits of Death Valley, Cali-fornia, where it is associated with soda-niter, NaNO3, and niter, KNOs(Palache et al.,195I, p. 310) ; 3) in saline material from caves in l imestone,Funeral Mountains (not previously reported; identified by Mrose inmaterial collected by James McAllister of the U. S. Geological Survey,1965); and supposedly a) in saline arid soil on the Roberts Massif,Shackleton Glacier, Antarctica (Claridge and Campbell, 1968). Osann(1894) reported on the morphology of darapskite from Oficinal Lautaro,which is between Oficina Flor de Chile and Oficina Alemania (Fig. 1).De Schulten (1896) described the synthesis of darapskite crystals.Larsen (1921,p.66) determined the optical properties of darapskite fromSanta Catalina, about 15 miles northeast of Oficina FIor de Chile.Sabelli (1967) recently determined the crystal structure of darapskitefrom a synthetic crystal.

    Fnlrurus ol rrrn Nrrnerr Dnposns

    The nitrate deposits are in the Atacama Desert of northern Chile; they extend fromabout lat. 19o30'S. to 260 S., a distance of neariy 700 kilometers (Fig. 1). Most are in aband less than 50 kilometers wide, between long. 69o30' W. and 70o W. The region is

    characterized by the broad Central Valley bordered on the west by the relatively low

    Coastal Range, where few peaks are more than 2000 meters in altitude, and on the east by

    the Andes Mountains, where peaks extend to altitudes of 6000 meters or more. The most

    extensive nitrate deposits are along the broad, debris-covered slopes on the east side of the

    Coastal Range, but deposits also are found in ciosed basins within the Coastal Range and

    on gentle slopes and in broad valleys along the lower Andean front.The nitrate deposits consist of caliche, generally not more than 2-3 meters in maximum

    thickness, in which diverse saline minerals (Table 1) occur as cement and impregnation inregolith and bedrock, and as associated high-purity veins. Cemented regolith is by far the

    most abundant typeof nitrate ore and has been the sourceof most of the nitrate produced

    in Chile. It is relatively dense and surprisingly hard and tough. High-purity-nitrate-bearingveins in bedrock and in saline-cemented regolith are found throughout the nitrate region,

    and they were an important source of the sodium nitrate produced during the 19th cen-tury. The veins are excellent sources oI mineralogical specimens and are still readily ac-cessible in the many localities where they were mined underground.

    The chemical composition and the mineralogy of the nitrate ore of Chile (Tables 1 and2) are unique and indicate an unusual environment of Iormation. The unique suite ofminerals results chiefly from the relatively high concentration of nitrate and iodate ions,which are sparse or absent in other saline deposits.'Ihe presence of these ions, togetherwith chromate, CrOrF, dicromate, Crqo?z-, and perchlorate, ClOr-, ions indicate an eviron-ment of extremely high oxidation potential. The sources of these ions are not yet known.

    Some may have been supplied by normal weathering processes and atmospheric fallout,but others may have formed at the sites of the deposits themselves. Extreme aridity andpaucity of microorganisms in the soil were probably important factors in the accumulationand preservation of the nitrate deposits.

    Several minerals, in addition to those listed in Table 1, have been reported in the liter-

    1 Oficina, as used in the nitrate fie1ds, refers to the site of the plant for treating nitrate

    ore.

  • 1502 GEORGE E. ERICKSEN AND MARY L,. MROSE

    Frc. 1. Index map of northern Chile, showing location of nitrate fields.

    ature but were not found in the material that we studied. Of these, only tarapacaite hasbeen identified as a component of tle nitrate ore (Brendler, 1923); t}le others either weremisidentified or were reported on the basis of reconstituted chemical analyses. Our studieshave revealed the presence of very small amounts of still otJrer minerals, eittrer mineralsthat heretofore have not been recognized as mineral species, or minerals for which pub-lished X-ray data are incorrect.

    OccunnnNcp oF DARApsKTTE rN THE NrrRArE DEposrrs

    Darapskite occurs in the nitrate ore as discrete crystals in cavities, asplaty masses in veins, and as granular material admixed with other

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  • DARAPSKITI')

    T,llr,a 1. S,lr.rNn MTNTnALS or rrIE Nrrn,lm Dnposrrs op

    Curr,r IorNrruED rN TrrE Pnnsamr Stunv

    1503

    Halite

    Soda-niterNiterDarapskite

    ThenarditeAnhydriteGypsumBassaniteGlauberiteKieseriteEpsomite

    NaClNaNOaKNOsNas(NOr)(SO) 'HrONa2SO4CaSOrCaSOr'2HzO2CaSOr.HrONazCa(SOr)zMgSOr.HzOMgSOr.THzO

    BloediteHumberstonite

    UlexiteProbertiteHydroboracile

    GinoriteKaliboriteLautariteDietzeiteLopezite

    NazMg(SOr)z'4HzOK3Na7Mg2(So{)s(Nor) r. 6Hzo

    NaCaBsOe'8H:ONaCaBsOg'5HrOCaMgB6Oq1.6H2OCazBuOzr 'SHrOHKMgzBrzOro(OH)m 4H:O

    Ca(IOs):caz(Ioa):(croDKz(CrzOr)

    Tenrs 2 CnrMrc.cr, Couposrtron ol WATER-SoLUBLE CoMPoNENTS rN NTTRATE ORES

    oI NoRTHERN Cnrr-e (CouposrrroN lN Wnrcnr Prncrxr')

    NOtc1-sol-clol-IOa-BrOl-Na+b

    K+Mgz+L a " '

    HzO

    6 . 3 53 . 8 76 6 0

    .035

    .061

    . 3 66 . 2

    . J O

    . 1 5r - l.)

    t . 7 4

    6 . 6 67 .07

    12 43.027.068. 6 1

    8 . 9. o r. 7 3

    2 . 2 71 .08

    t7 .671 1 . 1 13 . 4 7

    . J /

    .004

    .071 3 . 8

    . 9 5

    2 . 4 r20. 108 .841 . 0.03

    14.61 .44

    a 1

    1 . 5 02 . 7 6

    8 . 7 74 . 7 32 . 6 9

    . 4

    .042

    6 . 4. 2 8

    1 . 0 04 . 8 7

    1 Average grade of 5.97 million tons of nitrate ore treated at Oficina Pe&o de Valdivia

    during l2-month period, July 1, 1935-June 30, 1936. Furnished by Anglo-Lautaro

    Nitrate Corp.2 Average grade of 6.27 million tons of nitrate ore treated at Oficina Maria Elena, July

    1, 1951-June 30, 1952. Furnished by Angio-Lautaro Nitrate Corp.

    3 5 Channel samples of selected parts of nitrate ore layer from two localities at Oficina

    Humberstone. Analyses by Empresa Salitrera Victoria of samples collected by In-

    stituto de Investigaciones Geol6gicas of Chile.* Recalculated in part by present authors from original analyses to facilitate comparison

    o{ anaiyses from difie

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